\ REGULATION OF Till? CIRCULATION 103 



smaller, and so the vessel is constricted ; when they again 

 rdax the vessel dilates. The layer of plain muscular fibres of 

 the small blood-vessels can in this way regulate the supply of 

 blood to an organ. The vessels are supplied with nerves, 

 which are the motor nerves for these muscular fibres, corre- 

 sponding to the motor nerves for skeletal muscle fibres ; they 

 are called vaso-motor nerves. 



The plain muscular fibres of the vessels contract when they 

 receive impulses along these nerves, and they relax again when 

 these impulses cease. The nerve fibres which supply the 

 blood- vessels can be traced from the spinal cord, but it is 

 in the spinal bulb that the impulses governing the blood- 

 vessels are generated, and these pass down the spinal cord and 

 then out by several nerves to the small arteries of the body. 

 The part of the spinal bulb which in this way regulates 

 the calibre of the blood-vessels is called the vaso-motor 

 centre. The vaso-motor centre constantly sends out impulses 

 so as to keep the muscle fibres of the small arteries naturally a 

 little contracted, and consequently the vessels are kept in a 

 state of slight constriction. The vaso-motor centre keeps a 

 " rein " on the arteries, holding them in a condition of " tone " 

 as it is called. When an organ requires an increase of blood 

 supply, some influence, which varies in different cases, is 

 brought to bear on the vaso-motor centre, which leads to 

 a cessation of the impulses going out to the small arteries of 

 tin- particular organ, and in consequence the muscle fibres relax 

 and so the small arteries of the organ dilate. 



On the other hand, a decrease in blood supply is brought 

 about by more powerful impulses being sent to the muscle 

 fibres of certain vessels, so that they contract strongly, and as 

 a consequence the vessels are much constricted. 



Blushing 1 is caused by a dilation of the small arteries of 

 the face. This is brought about by the influence of some 

 emotion such as that of shame on the vaso-motor centre, leading 

 to the withdrawal of the impulses usually passing out to the 

 vessels of the fare. Sudden pallor, due to fear, is brought 

 about in a similar way, by an increase of the tonic constriction 

 of the small arteries of the face. The commonest cause of 

 sudden pallor, however, is a weakening of the heart, as occurs 

 in fainting. 



